There is no question whatsoever that some individuals are by nature violent. It is also no surprise that those individuals are more attracted to criminal activities. However it is interesting, that violence is not tied to some particular activity (like drugs or illegal arms sales), but rather to the fact that some activity is declared illegal. We can see that clearly on the example of the prohibition in the US: as soon as selling liquor became legal again, nobody in that particular business was killing each other.
But still, the question remains unanswered: why something being illegal induces violence? Surely it raises the price of conducting business, but it doesn't imply that people in that particular business should start killing each other.
The answer, in my view, lies in the fact that when something is made illegal, it draws line in people's mind. Once you start conducting an illegal business, you become a criminal. Once you become a criminal, what difference does it make if you simply graduate yourself from one type of criminal to another. Surely, punishments for violent crimes are usually harsher, but they're not harsh enough for people to not think about using violence. The choice people face is the following: okay, I'm already a criminal, if I use violence, I may get away with it too, I'll be able to win over my competitors and become more successful; if I don't, then the next guy will use violence against me.
Thus the answer to why criminals are violent is the threshold line. Whenever you set the threshold line of what is crime and what is not, you will see that many people operating right below it will inevitably be dragged lower and lower until they reach the point where they will have to use violence. So the non-violent and especially victimless crimes become gateways to more violent crimes.
Of course, the answer to this problem is all too known among reasonable people: repel the laws against victimless crimes. But the interesting question arises: where should the line be put? Or rather, if laws were created on the market, where would the market put the line between a criminal and a non criminal activity. And would it even make sense to draw such a line? So for example, would it be more beneficial (be it morally or economically beneficial) and would it decrease the total amount of violence if we let the family of the victim decide what punishment the murderer of their kin should have or to leave it up to some entity to which this family pays for protection? I'm asking this question because if we in principle can see that drawing arbitrary lines over what to consider a crime raises the total amount of violence, can we say this is always the case or is there some threshold under which it does make sense to draw such a line?